title: A computational study of ultrafast acid dissociation and acid-base neutralization reactions. II. The relationship between the coordination state of solvent molecules and concerted versus sequential acid dissociation.
abstract: We investigate the role played by the coordination state of pre-existing water wires during the dissociation of moderately strong acids by means of first-principles molecular dynamics calculations. By preparing 2,4,6-tricyanophenol (calc. pKa∼0.5) in two different initial states, we are able to observe sequential as well as concerted trajectories of dissociation: On one hand, equilibrium dissociation takes place on a ∼50 ps timescale; proton conduction occurs through three-coordinated water wires in this case, by means of sequential Grotthus hopping. On the other hand, by preparing 2,4,6-tricyanophenol in a hydration state inherited from that of equilibrated phenol (calc. pKa=7.6), the moderately strong acid finds itself in a presolvated state from which dissociation can take place on a ∼1 ps timescale. In this case, concerted dissociation trajectories are observed, which consist of proton translocation through two intervening, four-coordinated, water molecules in 0.1-1.0 ps. The present results suggest that, in general, the mechanism of proton translocation depends on how the excess proton is injected into a hydrogen bond network. In particular, if the initial conditions favour proton release to a fourfold H-bonded water molecule, proton translocation by as much as 6-8 Å can take place on a sub-picosecond timescale.
authors.affiliation: Département de Chimie, Université de Montréal, CP 6128, succursale Centre-Ville, Montréal, H3C3J7, Canada.
authors.forename: Patrick
authors.initials: P
authors.lastname: Maurer
authors.forename: Vibin
authors.initials: V
authors.lastname: Thomas
authors.forename: Radu
authors.initials: R
authors.lastname: Iftimie
mesh_terms: D000143:Acids / Q000737:chemistry*; D006860:Hydrogen Bonding; D056004:Molecular Dynamics Simulation; D015394:Molecular Structure; D011789:Quantum Theory; D014867:Water / Q000737:chemistry
chemical_list: D000143:Acids; D014867:Water